This invention relates to a multifrequency, broadband, dual-polarized or circularly polarized horn antenna.
Many applications require a highly efficient, multifrequency dual-polarized scanning antenna system. Ideally, the system should have the same constant width, symmetrical beam for both polarizations and all frequencies, and should have very low side lobes at all frequencies. A horn-reflector antenna system in disclosed is U.S. Pat. No. 3,949,404 capable of producing a spherical aperture phase front using a hyperbolic reflector illuminated by a corrugated conical horn. The farfield beam produced has low sidelobes and high efficiency. This antenna system is insensitive to frequency and polarization changes, and would therefore be suitable for multifrequency dual-polarized scanning purposes, but the problem is simultaneously feeding the corrugated conical horn with more than one frequency when the frequencies are spaced over one or more octaves.
Dual-frequency horn antennas have been devised in the past for widely spaced frequencies. The higher frequency is fed at the vertex of the horn in the usual manner, and the lower frequencies are fed into the horn where the cross-sectional dimensions are greater. See for example Hirayuki Kumazawa, Masaki Kayama and Yashio Kataoka, "Wide-Band Communication Satellite Antenna Using a Multifrequency Primary Horn", IEEE Transactions on Antennas and Propagation, May 1975, pp. 404-407. A problem with this approach is the limited number of widely spaced frequencies which can be accommodated with good isolation of the frequency channels. Thus, while such a multi-frequency antenna system may be adequate for the particular application for which designed, it would not be able to produce as many as five or more concentric distinct beams with similar width; and high efficiency over a wide range (e.g., 6.6 to 37 GHz). Such a requirement would be, for example, a scanning multichannel microwave radiometer (SMMR) to be used on the Numbus G and Seasat A satellites.
The SMMR requires a highly efficient multifrequency antenna system to be achieved by using a corrugated conical horn (CCH) and scanning reflector with geometry similar to that disclosed in U.S. Pat. No. 3,949,404. The reflector is an offset paraboloid of 31-inch diameter projected aperture fabricated out of graphite-epoxy, and the feed subassembly is a dual polarized horn. The design of the horn is an extension of a previous multi-frequency ring loaded CCH reported in a National Aeronautics and Space Administration Tech Brief NPO 13866 published in "Winter 1976 NASA Tech Briefs" at page 516. The horn is a broadband, dual-polarized antenna. The ring-loaded CCH has been reported by Fumio Takeda and Tsutomu Hashimoto, "Broadbanding of Corrugated Conical Horns," IEEE Trans on Ant. & Prop., Nov. 1976, pp. 786-792. See also U.S. Pat. No. 3,754,273 granted to the authors as coinventors with Yoshihiro Takeichi. The problem of adapting a ring loaded CCH antenna as a feedhorn in an SMMR, or other application requiring a highly efficient, multifrequency dual polarized antenna, is to obtain operation on both polarizations at a multiplicity (typically 5) of frequencies over a wide range (typically from 6 to 37 GHz) to provide a multiplicity of signals (two for each frequency, one for each polarization) simultaneously, with low insertion loss and good isolation between frequencies and polarizations, as well as extremely low sidelobes and symmetrical nearly equal beamwidths for all polarizations and frequencies. It is that problem that is solved by this invention.